21. Biochemistry | Serum Enzymes

What is the definition of an isozyme (isoenzyme)?

Variant forms of the same enzyme that catalyze the same chemical reaction in diff tissues/organs

What is the structural feature of isozymes?

Composed of different amino acid sequences → differing kinetic properties.

What is the functional characteristic of isozymes?

They catalyze the same reaction but are optimized for specific tissue environments.

Where are isozymes typically expressed?

Isozymes are expressed in different tissues or organs, showing tissue-specific expression.

What is the subunit composition of Lactate Dehydrogenase (LDH)?

LDH is a tetramer composed of H (heart) and M (muscle) subunits forming five isoforms: H4, H3M1, H2M2, H1M3, M4.

Which LDH isoform is H4 and where is it found?

LDH-1 (H4) is primarily found in the heart (oxygen-rich tissue).

Which LDH isoform is M4 and where is it found?

LDH-5 (M4) is found in the liver and skeletal muscle (oxygen-poor tissue).

Which LDH isoforms are associated with acute leukemia?

LDH-2 (H3M1) and LDH-3 (H2M2) are commonly seen in acute leukemia.

Which LDH isoform is the fastest in SDS-PAGE and is heat stable?

LDH-1 (H4) migrates the farthest (fastest) in SDS-PAGE and is heat stable.

Which LDH isoform is the slowest in SDS-PAGE and is heat labile?

LDH-5 (M4) migrates the least (slowest) in SDS-PAGE and is heat labile.

What is the function of Creatine Kinase (CK)?

Creatine Kinase catalyzes the formation of creatine phosphate, a high-energy molecule.

What are the three isozyme types of Creatine Kinase?

CK1 = CK-BB (Brain), CK2 = CK-MB (Heart muscle), CK3 = CK-MM (Heart + Skeletal muscle).

What is the composition of heart muscle CK isoforms?

Heart muscle contains 1/3 CK-MB (CK2) and 2/3 CK-MM (CK3).

What are the sources of serum enzymes in the blood?

Normal secretion (e.g., liver coagulation factors, AAT), cellular turnover, and tissue damage releasing enzymes like ALT, AST, LDH, CK.

Which enzymes indicate liver damage?

Alanine aminotransferase (ALT) and Aspartate aminotransferase (AST).

Which enzyme is elevated in cholestasis and bone disorders?

Alkaline phosphatase (ALP).

Which enzymes are elevated in muscle damage or rhabdomyolysis?

CK-MM, CK-MB, Aldolase, aspartate aminotransferase

Which enzymes are diagnostic for myocardial infarction?

CK-MB and Troponin I/T (Troponins are the gold standard).

Which enzymes indicate pancreatic injury?

Amylase and Lipase.

Which enzymes help determine tissue damage in RBC or hemolysis?

Lactate dehydrogenase (LDH) isoforms.

When do cardiac Troponin I/T levels rise, peak, and normalize after MI?

Rise: 4-6 hrs, Peak: 24-36 hrs, Normalize: 3-10 days.

When do CK-MB levels rise, peak, and normalize after MI?

Rise: 4-8 hrs, Peak: ~24 hrs, Normalize: 2-3 days.

Why is CK-MB preferred over CK-MM for MI diagnosis?

CK-MB is specific to heart muscle, whereas CK-MM is also present in skeletal muscle.

How does SDS-PAGE separate proteins?

It separates proteins based on molecular mass; smaller proteins migrate farther. SDS denatures proteins and gives them a negative charge.

What is the role of beta-mercaptoethanol or DTT in SDS-PAGE?

They reduce disulfide bonds to fully denature proteins.

How does gel filtration chromatography separate proteins?

It separates proteins based on size; larger proteins elute first, smaller proteins are retained longer in pores.

How are proteins detected after SDS-PAGE or chromatography?

Using Western blotting (antibodies) or mass spectrometry.

Which serum enzymes are used for diagnosing the liver (i.e. obstructive jaundice)?

Liver: Alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), gamma-glutamyl transferase (GGT), 5'-NT

Which serum enzymes are used for diagnosing the heart?

Heart: CK-MB, Troponin I/T

Which serum enzymes are used for diagnosing mm.?

Muscle: CK-MM, Aldolase

Which serum enzymes are used for diagnosing the pancreas?

"

Pancreas: Amylase, Lipase"

Which serum enzymes are used for diagnosing bone (i.e. cholestasis, rickets)?

Bone: Alkaline phosphatase (ALP)

Which serum enzymes are used for diagnosing RBCs?

Blood: LDH isoforms.

A 73-year-old female was admitted to the hospital for severe chest pain 4 days ago. She was found to have an ST-elevation MI, was treated with percutaneous coronary intervention, and has done well. She is almost ready for discharge when she starts to complain of more chest pain. She says it feels similar to the pain she had 4 days ago. Which biomarker is best to make your diagnosis?

A. CK-MB

B. Troponin C

C. Troponin I

D. Troponin T

A

The correct answer is CK-MB (A). The patient is already admitted with an MI. Her symptoms are consistent with reinfarction after a primary MI. CK-MB levels normalize 3 days after an MI, so an elevation in levels of CK-MB 4 days after a primary MI indicates a reinfarction. Troponin C (B) is not specific to cardiac myocytes, so it is not a good biomarker for cardiac ischemia. Troponin I (C) and troponin T (D) levels do not normalize until 10 days after the primary MI, so they are not useful in diagnosing a reinfarction.

Which of the following biomarkers have lower specificity for diagnosing MI compared to the others listed?

A. CK-MB and troponin C

B. CK-MB and troponin I

C. CK-MB and troponin T

D. Troponin C and troponin I

E. Troponin I and troponin T

A

The correct answer is CK-MB and troponin-C (A). CK-MB is very sensitive for MI diagnosis, but lacks specificity, since it can also increase in chronic kidney disease, cardiac trauma and surgery, and cardiac inflammation. Troponin-C is largely found in myocytes outside the heart, so would be neither specific nor sensitive for MI diagnosis. CK-MB and troponin I (B), CK-MB and troponin T (C), troponin C and troponin I (D), and troponin I and troponin T (E) are quite sensitive for MI diagnosis.